1. Field of the Invention
This invention pertains generally to hybrid powered vehicles employing both electric motors and auxiliary power units, and more particularly to controlling fuel consumption in a charge depletion hybrid electric vehicle.
2. Description of the Background Art
As discussed in my copending U.S. application Ser. No. 08/963,067, a hybrid electric vehicle (HEV) is a vehicle with electricity as the primary energy source and an auxiliary power unit (APU) as the secondary source. The APU is typically an internal combustion engine (ICE) utilizing reformulated gasoline, methanol, ethanol, diesel, compressed natural gas or other hydrocarbon as a fuel source. The electrical energy is stored in chemical storage batteries or capacitors. A series hybrid electric vehicle uses the ICE to drive a generator which supplies power to the electric motor (EM) or charges the batteries, whereas a parallel hybrid uses the ICE and EM together to directly drive the wheels. In both configurations, the ICE is used to supplement the energy capacity and power capability of the battery pack.
Most parallel hybrid vehicles are configured with a large ICE. The EM is selected for urban driving and acceleration, since the ICE may not be used provide the power required for this driving demand. In emergency situations, the ICE can provide "limp-home" capability when the batteries reach a depth of discharge (DOD) where the EM can no longer accelerate the vehicle.
Various control strategies have been previously developed for operating the EM and ICE in hybrid vehicles. None of the existing control schemes, however, integrate the operation of the EM and ICE in a hybrid vehicle in a way which maximizes both efficiency and range on the batteries while maintaining performance of the vehicle; that is, using a "charge depletion" control method in accordance with the present invention. Conventional control schemes operate on the principle of sustaining the charge on the batteries, rather than depleting the batteries as in the present invention. The present invention satisfies the need for a control method and apparatus that will provide for integration of the EM and ICE in an efficient manner without sacrificing vehicle performance.
Furthermore, government regulating agencies such as the Environmental Protection Agency (EPA) and the California Air Resources Board (CARB), as well as users of a "charge depletion hybrid electric vehicle" (CDHEV) in accordance with the present invention, may want to change the relative amounts of liquid or hydrocarbon fuel and electric energy used by a fleet of CDHEVs, since the cost and technology of each commodity may change over time. For example, electric energy is currently less expensive for powering a vehicle per mile than gasoline. On the other hand, emissions from electrical powerplants may be less clean and less environmentally friendly than from gasoline engines. The present invention further satisfies the need to provide for changing the manner in which operation of the EM and ICE are controlled so as to allow for adjustment of the relative amounts of combustible fuels and electric energy consumed during operation of the vehicle fleet.
The EPA currently specifies to vehicle manufacturers what is referred to as a "corporate average fuel economy" (CAFE) as well as individual vehicle emissions. A CDHEV in accordance with the present invention permits the EPA to also specify and regulate both emissions and fuel consumption for a fleet of hybrid electric vehicles. On the other hand, local regulations may be different since air quality and performance requirements change depending upon traffic and road infrastructure.